The present invention relates to low density, flexible microcellular elastomers suitable for the preparation of molded shoe soles, shoe inners, and midsoles; to an isocyanate-reactive component useful for the production of such microcellular elastomers; to an isocyanate-terminated prepolymer useful for the production of such microcellular elastomers; and to a process for the production of such microcellular elastomers from such isocyanate-reactive component and/or isocyanate-terminated prepolymer in which (1) carbon dioxide is dissolved in one or both of the polyurethane-forming reaction mixture components in an amount sufficient to a froth density in such component or components of from about 0.1 to about 0.8 g/cc and (2) the amount of dissolved carbon dioxide plus carbon dioxide generated during the isocyanate/water reaction is sufficient to produce a polyurethane-forming reaction mixture having a free rise density of from about 0.03 to 0.3 g/cc.
Cushioned soles for footwear, particularly athletic footwear, are generally prepared from EVA (ethylenevinylacetate) microcellular foams. The processing of such foams is not straightforward, and the foams themselves do not have optimal properties. However, such foams continue to be used due to their availability in the very low density range, i.e., 0.1 g/cc to 0.35 g/cc.
Polyurethane polymers generally exhibit physical properties which are superior to EVA polymers. However, numerous difficulties arise when attempts are made to mold polyurethane microcellular elastomers at low densities. Due to the hardness required for the end use, considerable amounts of low molecular weight chain extenders are required. In microcellular foams which are water-blown, the urea short segments which are created cause the formulations to have poor processability, resulting in both shrinkage and splits in the parts. The physical properties are also compromised. These problems have prevented use of low density (<0.35 g/cc) polyurethane microcellular elastomers, and more particularly, very low density (<0.30 g/cc) polyurethane microcellular elastomers.
One of the most commercially important applications for microcellular polyurethane elastomers is production of shoe soles. These elastomers are generally produced from an isocyanate-terminated prepolymer, a polyol, a chain extender, a blowing agent and a surfactant. The properties of these microcellular elastomers are generally attributed to the specific prepolymer used to make the elastomer. (See, e.g., U.S. Pat. Nos. 5,246,977 and 5,849,944.)
U.S. Pat. No. 6,458,861 discloses carbon dioxide blown low density, flexible microcellular polyurethane elastomers which have a more uniform cell structure and enhanced physical properties than the previously known microcellular elastomers. These improvements are attributed to dissolution of the carbon dioxide blowing agent in one or both of the polyurethane-forming reaction components. Where water is included as a second blowing agent, U.S. Pat. No. 6,458,861 teaches that the water should be used in an amount which is less than 50% by weight of the total amount of water necessary to produce an all-water blown microcellular elastomer having the same density.